1. Field of the Invention
The present invention relates to a frequency shift keying (FSK) signal modulator for producing an FSK signal. More particularly, the present invention relates to an FSK signal modulator connectable to an FSK demodulator for receiving an FSK signal to perform digital signal processing thereon to produce a binary FSK signal.
2. Description of the Background Art
When an FSK signal corresponding to a digital signal resulting from quantization of a bit is to be received by an FSK demodulator, it is necessary for an FSK signal modulator connectable to the FSK demodulator to produce such an FSK signal in the form of binary signal corresponding to one bit. This type of FSK signal modulator has been disclosed in the Japanese patent laid-open publication No. 231337/1995.
However, an FSK demodulator adapted for receiving a binary input signal output from an analog receiver circuit and processing the signal in digital cannot accept an FSK signal transmitted from an FSK modulator adapted for processing in digital an input signal to produce the FSK signal in the form of multiple-value information, corresponding to plural bits or multi-bits when quantized. Hence, an FSK signal modulator capable of communicating with an FSK demodulator has to be provided not only with an FSK baseband modulator for producing a baseband modulated signal in the form of multiple-value or multiple-bit information but also with a frequency converter for converting the frequency component of the output signal of the modulator and with a one-bit converter for converting the frequency component into signals each corresponding to one bit.
Such a frequency converter further needs to be provided with a frequency generator for generating a multi-value frequency signal, providing the frequencies necessary for the one-bit conversion, and a multiplier for multiplying the multi-value output signal of the FSK baseband modulator with a multi-value frequency signal. With this multiplier, it is necessary to carryout two multiplication operations, that is, the multiplication operations on the real-number and imaginary-number axes. Thus, with the conventional FSK signal modulator, the circuit is increased in size, while difficulties are met in simplifying the circuit constitution.